1. Field of the Invention
The present invention relates to a laser method and apparatus, and, more particularly, to such a method and apparatus in which a metal vapor, heated by electric discharge, is used to radiate a laser beam by exciting the metal vapor.
2. Description of the Related Art
A structure of a conventional metal vapor laser apparatus 101 is shown in FIG. 1. As shown, this type of metal vapor laser apparatus 101 includes a cylindrical vacuum container 102 having a heat insulating material 103 and a plasma tube 104. A pair of discharge electrodes 105 and 106 are set at each end of the plasma tube 104. A discharge chamber 107, containing a buffer gas fill, is surrounded by the plasma tube 104 and the discharge electrodes 105 and 106.
The discharge electrodes 105 and 106 are connected to a power supply 108. Further, the vacuum container 102 is sealed hermetically by windows 109 and 110 closing both ends of the vacuum container 102. Further, a resonator, comprised of an output mirror 111 and a total reflective mirror 112, is provided outside of the windows 109 and 110. In addition, lumps 113 of a laser medium metal are provided within the plasma tube 104.
As used herein and in the appended claims, the term "lump" means a piece of metal having an indeterminate size and shape. Generally, the size of a lump used in the metal laser art approximates one cubic centimeter but precision in this respect is not necessary. Also, it is common practice to use about 3-5 such lumps in the tube 104, but the number may vary depending on the size of the tube.
Each end of the vacuum container 102 is connected with a buffer gas supply device 114 and a vacuum pump 115, respectively. Buffer gas (rare gas such as neon, helium, etc.) is supplied into the vacuum container 102 by the buffer gas supply device 114 and the buffer gas in the vacuum container 102 is exhausted by the vacuum pump 115.
When a laser beam is generated by the metal vapor laser apparatus 101 described above, a buffer gas is supplied into the vacuum container 102 by the buffer gas supply device 114 and then exhausted from the vacuum container 102 by the vacuum pump 115 in a manner to maintain the chamber 107 under low pressure. Voltage is applied between the discharge electrodes 105 and 106 from the power supply 108, and a pulsed discharge occurs in the discharge chamber 107. The plasma tube is heated and the metal lumps 113 in the tube, for example, gold or copper lumps, are also heated to within the range of 1500.degree. C.-1700.degree. C. and converted to a metal vapor. The metal atoms in the vapor state are excited by the glow discharge in the plasma tube 104, and the laser beam is developed at the mirrors 111, 112 of the resonator.
To increase the laser beam output of such conventional apparatus, it has been the practice to increase the diameter of the bore of the discharge chamber 107 in recent years. However, if the diameter size of the bore of the discharge chamber 107 (hereinafter referred to simply as "bore") becomes about 60 mm or more, (i.e., the cross sectional area is 25 cm.sup.2 or more), intensity near the center of the laser beam decreases. If the bore becomes about 80 mm, this phenomenon becomes more and more conspicuous. This phenomenon is generally referred to as a "annular beam" and occurs because, in a metal vapor laser device with the discharge chamber 107 having a large bore, the temperature of gas around the center of the discharge chamber 107 is generally higher than that of the nearby wall of the plasma tube 104 by more than 1000.degree. C. As a result, the density of the lower level atoms in the center is increased relative to that of the nearby wall and the gain in the center part is reduced to produce the annular beam.
As the bore of the discharge chamber 107 is increased, the ratio of laser beam output to input electrical power, i.e., energy efficiency, tends to decrease. Generally, the laser beam output is increased with discharge power. When the electrical discharge power is especially high, or the bore of the plasma tube is very large, or cycle frequency is high, the decrease in laser beam output efficiency is remarkable.
With respect to the bore, the annular beam phenomenon begins to occur when the bore size is about 60 mm and increases remarkably when the bore size is about 80 mm. The cause of the phenomenon is that, when the temperature at the center of the discharge chamber 107 is high, the metal atomic density at lower oscillation levels becomes higher at the center of the chamber 107 and lowers the intensity of the laser beam in that region.
The annular beam phenomenon is represented in FIG. 2 by a curve resulting from a plotting of laser output power against time of laser operation. As shown, after (H) hours of laser operation, a peak power (P) of the laser beam is developed. However, thereafter the power is gradually reduced to a constant power (L) which is significantly lower than peak power (P).
A report of a study that was tried to prevent the annular beam phenomenon and increase the output by deexciting the lower level metal atoms by mixing molecular gas such as hydrogen, etc. in buffer gas to have molecular gas collide with the lower level metal atoms in the small bore laser device (the direct diameter 32 mm) is found in an article by Zhen-Guao Huang et al.; Japanese Journal of Applied Physics Vol. 25, No. 11, 1986 pp. 1677-1679. According to this study, an annular beam in the green spectrum and generated in a small bore was improved and an increase of laser output power was observed at a mixing ratio below 1.8% of hydrogen gas to neon gas, as the buffer gas.
However, in the article reporting the above study, nothing was described about improving the ratio of a green beam line and a yellow beam line (G/Y ratio) to evaluate the annular beam phenomenon. When the G/Y ratio is large (i.e. the green beam lines are greater in number than the yellow beam lines), the annular beam increases at a lower rate, but when the G/Y ratio is small (i.e. the green beam lines are less in number than the yellow beam lines), the annular beam increases at a high rate of increase and the intensity of the oscillated laser beam at the center axis decreases. The relation between G/Y ratio and the annular beam phenomenon is found in the reported study. The value of G/Y ratio, however, is used as a measurement for the annular beam phenomenon. There is no description about improving the G/Y ratio to reduce the annular beam.
Further, in the above study, the laser output efficiency per applied power is as low as about 0.5% and is not an efficiency at the practical level (about 1 to 1.5%).
Laser output power and laser efficiency depend on the temperature of the metal lumps used as the laser medium because the metal vapor density is determined by the temperature of the metal lumps. Therefore, the temperature of the metal lumps must be controlled to maintain the metal lumps at the optimum temperature to supply the metal vapor in an optimum amount. Only in this way can the laser beam be obtained efficiently at higher laser output power. To control the temperature to the optimum value, a material for the thermal insulator, or the thickness of that thermal insulator must be established for the point of the heat transfer rate and the input energy to the plasma in order to optimize the temperature of the plasma tube.